One type of reactor processes wafers in batches where the wafers of a batch are simultaneously subject to the same treatment. Another type of reactor processes wafers individually. The latter type of reactor is typically used to process larger wafers, i.e., wafers that have a diameter of about 200 millimeters or 300 millimeters. A floating wafer reactor, as described in U.S. Pat. No. 6,183,565, for example, processes a single wafer at a time. Current state-of-the-art technology is configured for 300-millimeter wafers; future technology is expected to employ even larger substrates.
The reactor described in the '565 patent is a “hot wall” reactor having an upper part and a bottom part that form a process chamber and that include heating elements to heat the process chamber and the wafer to a predetermined temperature. The upper and bottom parts are relatively massive, such that a stable temperature is reached for the entire chamber, relatively unaffected by the loading of cold wafers. A controller controls the heating elements so that the actual temperature of the reactor is the same as a predetermined temperature selected for a particular process step. Within the process chamber, the wafer is supported upon gas cushions (“floating”) at a very short distance from upper and lower walls of the process chamber by gas flows in opposing direction from the upper and lower walls.
While the wafer is in the process chamber, the wafer is subject to a variety of processing options. In one option, the wafer is subjected to one or more stages of heat treatment such as annealing, during which the wafer is exposed to an inert gas (N2, Ar, He) only. In another option, during treatment the wafer is exposed at least part of the time to a reactant gas such as an oxidizing gas (O2 or H2O, N2O, CO2) or a nitridizing gas (NH3, N2, depending on the temperature). In yet another option, the treatment can include chemical vapor deposition (CVD).
The environment in which the reactor is placed, for example, a clean room of a laboratory or a semiconductor fabrication plant, is usually at room temperature. That is, at the beginning of the wafer processing or at the beginning of one of the processing stages, a handling apparatus moves the upper and lower parts apart to open the reactor and loads the wafer horizontally into the process chamber. By moving the upper and lower parts towards each other such that the wafer is at a very short distance from the upper wall and the lower wall, the wafer is heated very quickly and is then exposed to a very high temperature, for example, 1000° C. during annealing, compared to the room temperature.
U.S. Pat. No. 6,329,304 and Dutch application No. 1018086, both assigned to applicant, describe methods and apparatuses to achieve a reproducible treatment for a series of substrates. U.S. Pat. No. 6,329,304 describes that when a wafer is loaded into the process chamber of a floating wafer reactor, the surface temperature of the walls facing the wafer drops by about 10° C., whereas the interior temperature of the walls, i.e., further away from the wall surfaces, drops by about 3° C. Although the reactor's temperature control can compensate for this unequal wall temperature, a resultant time delay is undesirable for certain processes. Therefore, U.S. Pat. No. 6,329,304 discloses applying a pulse of energy to the heating elements during loading the wafer in order to heat the walls for a short period of time independently from the temperature sensors. The additional heating during that time is intended to compensate for the temperature drop.
In Dutch application No. 1018086 a more sophisticated method is described. According to the method described, a substrate is loaded when a desired starting temperature is measured in the reactor wall, close to the wall surface facing the wafer. The heat transfer to the wafer results in a drop in measured temperature, followed by a recovery. The substrate to be treated is removed from the reactor before the starting temperature is reached again whereas the next substrate is loaded at the moment the starting temperature is reached again. In particular for very short process times, this methods helps to achieve a reproducible thermal budget for each one of a series of substrates to be treated sequentially.
Reactors configured to perform a thermal treatment are typically provided with a plurality of heating zones. The purpose of these multiple heating zones is to achieve a uniform temperature inside the reactor so that a substrate receives a uniform treatment over its entire surface. Parts of the reactor that are located in the periphery of the reactor suffer from a larger heat loss than parts of the reactor that are more centrally located. Consequently, more power needs to be supplied to heating zones that are located near the periphery to compensate for this heat loss and to achieve the desired uniform temperature.
A problem encountered during operation of the floating wafer reactor as described above is that for short anneals, with an anneal time of the same order of magnitude as the unloading time of the substrate, a non-uniform process result over each wafer was achieved whereas the temperatures were within the control limits and uniform over the wafer. Interestingly, the process result, in this case the resistivity of the substrate, showed a distinct unidirectional trend in a direction parallel to the direction of unloading of the wafer from the reactor. In other cases, the resistivity over the wafer showed a radial gradient. When increasing the anneal time, these gradients disappear but then other substrate properties are affected in an unacceptable way. In certain embodiments or applications, the very short anneal times are mandatory. It is contemplated that temperature gradients during removal of the substrate from the reactor influence the process result in a significant and undesirable way.
It is an object of the present invention to provide a method and apparatus for processing a substrate that allows very short processing times whereas the disadvantage of a non-uniform process result is avoided.
In accordance with one aspect of the invention, a reactor is provided for heat treatment of a flat substrate. The reactor includes a heated body, having a substantially flat surface facing a flat substrate during processing. A substrate handling mechanism is configured to place the flat substrate to be processed parallel to and in close proximity to the substantially flat surface of the heated body, and configured to remove said substrate in a removal direction from the heated body after processing. A plurality of heating elements are associated with the heated body and arranged to define heating zones connected to a controller. The controller is configured to control the heating elements, while the controller and heating zones are configured to provide for a non-uniform temperature laterally across the flat surface of the heated body.
In accordance with another aspect of the invention, a method of operating a thermal reactor is provided for the treatment of flat substrates. The method includes loading a substrate into the reactor. Heating elements are selectively operated to define a non-uniform temperature distribution across a heated body adjacent the substrate, which distribution extends in a lateral direction over the substrate and is selected to compensate for an uneven thermal effect upon the substrate during operation of the reactor. The substrate is processed for a predetermined period of time while the substrate is subject to the non-uniform temperature distribution. The substrate is unloaded from the reactor after the predetermined period of time.
In accordance with a further aspect of the invention, a reactor for heat treatment of a flat substrate is provided. The reactor includes a substrate enclosing structure defining a process chamber between an upper part and a bottom part. The upper and bottom parts are configured to separate for loading and unloading a flat substrate along a loading/unloading direction. A support structure is configured to position the substrate between the upper part and the bottom part. The substrate has major surfaces within about 2 mm of each of the tipper part and the bottom part within the process chamber during processing. A plurality of heating elements is arranged to define heating zones, each extending over only a portion of upper and bottom parts. A controller is connected to the heating elements individually, the controller being programmed to provide a non-uniform temperature distribution across at least one of the upper and lower parts.